As per manual, the command ATAG000000000000FFFE should update the Aggregator Address on routing table, the question is:
Considering 900HP is on API mode, Is it expected an Aggregate addressing update message on all the modules that received the AG update?
http://ftp1.digi.com/support/documentation/90002166_C.pdf (pag 68 - Building Aggregate Routes)
If I understand your question, all nodes that are within the Broadcast hops listing in the network should update their address table.
Yes, you should see all nodes that are in API mode output an Aggregate Addressing Update API packet if they receive an Aggregate route update.
As an example, I setup 3 radios. 2 were in AT mode, the other in API. All radios were set to default with the AT/API mode being the exception.
I sent an ATAGFFFF from one of the AT radios. The API radio output an Aggregate Addressing Update API packet while the AT radio did not output anything.
Both radios, however, did change their DH/DL to the address of the sending radio.